The animation can contain anything that looks good: lines, blurry waves, etc. Something like this is what would be cool: . At its best it shouldn't only animate around the hands, but also the body as it moves or the silhouette's head.

for your version you will probably want to track the hand of the silhouette. ae's built in tracker should work fine for that, the manual cover tracking fairly well and you can find many tutorials for tracking.

the trickier part will be having the emitter (or particle producer) move in z-space. your track will only be in 2 dimensions (x,y) so the z-position will need to be animated by hand.

to get the particle system to actually move behind the silhouette, there are two methods. particular has a obscuration layer feature which will allow this, but has a few draw backs. and then there is z-clipping which would involve splitting the system onto two (or more) layers and sandwiching the silhouette between them.